Biological treatment of spent caustics

ABSTRACT

A process for the biological treatment of a spent caustic solution containing sulphides is disclosed, wherein the solution is introduced into an aerobic reactor containing sulphide-oxidising bacteria, and the sulphides are partly converted to elemental sulphur and partly to sulphate by controlling the redox potential in the reactor at a value below -300 mV (against an Ag/AgCl reference electrode), or below -97 (against a H 2  reference electrode). Also disclosed is process for the biological treatment of an aqueous solution containing sulphides and/or mercaptans in an aerobic reactor, wherein the solution is treated in the presence of bacteria of the new species Methylophaga sulfidovorans.

FIELD OF THE INVENTION

The invention relates to the biological treatment of spent causticsresulting from the removal of sulphides from hydrocarbon sources.

BACKGROUND

Sodium hydroxide solutions are used in petroleum refining and chemicalindustries to remove hydrogen sulphide from various hydrocarbon streams.When the sulphide has reacted with the sodium hydroxide, the resultingsolution is usually referred to as spent sulphidic caustic. Depending onthe hydrocarbon source, spent caustics may also contain phenols,mercaptans, amines and other organic compounds that are soluble oremulsified in the caustic. Spent caustics typically have a pH greaterthan 12 and sulphide concentrations exceeding 2 wt. % (≈a more than 0.6mol/l).

At the moment, spent caustics are usually treated by the "wet airoxidation", wherein sulphides and mercaptans are oxidised chemically athigh pressures and temperatures. This process is expensive because ofthe required chemicals, and leads to residual waste in the form ofgaseous sulphur dioxide and liquid sulphuric acid and sulphate. Anotherknown method of disposal of spent caustic is deep well injection, whichis also expensive.

A biological process for the treatment of spent caustics was describedby Rajganesh, Sublette, Camp and Richardson, Biotechnol. Prog. 1995(11), 228-230. In this process, sulphides are completely oxidised tosulphate by Thiobacillus denitrificans. However, the production of onlysulphate is often not desirable because the pH may become too low. Thisknown process also requires nitrate, which has to be added to the spentcaustics, leading to additional costs for chemical requirements.

SUMMARY OF THE INVENTION

It has been found now that the biological treatment of spent causticsand similar waste streams can be improved by controlling the redoxpotential of the biological treatment medium so as to partly orpredominantly produce elemental sulphur in addition to sulphate.

It has furthermore been found that the treatment of spent caustics alsocontaining mercaptans can be accomplished by using bacteria of the novelstrain Methylophaga sulfidovorans.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect, the invention pertains to a process for thebiological treatment of a spent caustic solution containing sulphidesand possibly other sulphur compounds, wherein the sulphides are partlyconverted to elemental sulphur and partly to sulphate.

The biological oxidation of sulphide by aerobic Thiobacilli can berepresented by the following equations, especially (1) and (2):

    HS.sup.- +0.5O.sub.2 →S°+OH.sup.-            (1)

    HS.sup.- +2O.sub.2 →SO.sub.4.sup.2- +H.sup.+        (2)

    2HSCH.sub.3 +7O.sub.2 →2H.sub.2 SO.sub.4 +2CO.sub.2 +2H.sub.2 O(3a).

In reaction (1) sulphur is formed with an increase in pH, whereas inreaction (2) sulphate is formed with a drop in pH. In order to cause theoxidation to proceed partly or pre-dominantly through reaction (1), onecould in principle use a low oxygen concentration, since the higher theoxygen concentration is, the more sulphate is formed. However, atmoderate sulphide loading rates (up to about 250 mg/l.h), sulphateproduction is already complete at oxygen levels as low as 0.1 mg/l,which is about the detection limit. One could also use a high sulphideload, but this leads to increased mercaptan loads which are difficult tobiodegrade.

It was surprisingly found, that the sulphide oxidising reaction can becontrolled towards partial or predominant sulphur formation by adjustingthe redox potential of the medium of the oxidation at a value below -300mV (against an Ag/AgCl reference electrode). The following redox rangeswere found: at a redox potential below -360 mV (against an Ag/AgClreference electrode), hydrogen sulphide is completely converted toelemental sulphur; at a redox potential between -360 and -300 mV(against an Ag/AgCl reference electrode), hydrogen sulphide is partiallyconverted to elemental sulphur and partially to sulphate; at a redoxpotential above -300 mV (against an Ag/AgCl reference electrode),hydrogen sulphide is completely converted to sulphate. The range of-300/-360 mV against Ag/AgCl corresponds to a range of -97/-157 mVagainst a H₂ reference electrode at 30° C. This allows an optimumsulphide removal combined with an effective neutralisation of the spentcaustics, so that they can be safely discharged.

Preferably the redox potential is controlled such that the sulphatelevel produced results in neutralisation (pH 6-9) of the spent caustics.The preferred redox potential range is from -300 to -390 mV, morepreferably from -320 to -360 mV, optimally from -340 to -350 mV (againstAg/AgCl, corresponding to -137 to -147 mV against H₂), at 30° C., pH=8.A more detailed description of the process control using the redoxpotential is given below.

In order to avoid too high local pH's in the aerobic bioreactor, theincoming spent caustics solution should be well dispersed in the reactormedium. As a useful alternative, the spent caustics can be diluted witheffluent from the aerobic reactor to lowers its pH. This can be donee.g. in a premixing tank. The oxidation can be carried out usingsulphide-oxidising bacteria of the genera Thiobacillus, Thiomicrospiraand related microorganisms. The bacteria can be used as such, or on adispersed carrier, or can be immobilised on a solid carrier. Theeffluent can be treated in a conventional manner: elemental sulphur canbe separated off by settling or centrifuging and be reused or burnt.Sulphate can be discharged and thiosulphate, if any is formed, may bebiologically converted to other sulphur compounds.

In another aspect, the invention concerns the use of a novelsulphur-oxidising bacterium in the treatment of waste water containingmercaptans and/or other sulphur compounds. The novel Gram-negativebacterium has been named Methylophaga sulfido-vorans. The strain RB-1thereof was obtained from a microbial mat from an estuarine intertidalregion (Oosterschelde, the Netherlands). A sample has been deposited atthe Delft Culture Collection (LMD 95.210). It exhibits 98.8% and 98.3%16S rRNA similarity with Methylophaga thalassica (DSM 5690), and M.marina (DSM 5689), respectively. The bacteria are irregular, oval-shapedrods of 0.2 by 0.9 μm, having polar flagellum. Catalase and oxidase arepositive. The GC content of the bacterial DNA is 42%. Optimumtemperature is 22° C. growth occurs between 17 and 35° C. Optimum pH isbetween 7 and 7.5. They use the ribulose monophosphate route for carbonassimilation. It is an obligately methylotrophic organism.

Methylophaga sulfidovorans grows on methanol, formaldehyde, methylamine,dimethylamine, dimethyl sulphide, methyl mercaptan and hydrogensulphide. No growth was detected on trimethylamine, methane andmethanesulphonic acid. It cannot oxidise thiosulphate, elemental sulphurand formate.

The oxidation of methyl mercaptan and dimethyl sulphide is proceedsalong the following equations:

    2 HSCH.sub.3 +5 O.sub.2 →H.sub.2 S.sub.2 O.sub.3 +2CO.sub.2 +3H.sub.2 O                                               (3b)

    2 CH.sub.3 SCH.sub.3 +8 O.sub.2 →H.sub.2 S.sub.2 O.sub.3 +4 CO.sub.2 +5H.sub.2 O                                               (4)

The bacteria of the species Methylophaga sulfidovorans are usedaccording to the invention for converting hydrogen sulphide, mercaptansand organic sulphides like methyl mercaptan, dimethyl sulphide anddimethyl disulphide, to higher oxidised species, in particularthiosulphate. Thus the invention pertains to any process of biologicalsulphur removal wherein the new species is used.

The Methylophaga sulfidovorans can be used in generally the same way asconventional sulphide-oxidising bacteria. It is preferred, however thatthe salinity of the aerobic reactor is close to the value of seawater,which means a salt concentration (NaCl) between about 30 and 40 g/kg, inparticular between 33 and 37 g/kg. If the solution to be treated has asubstantially different salinity, this can be adapted by the addition ofsodium salts or by dilution, e.g. with tap water, as the case may be. Inparticular, the sodium concentration in the aerobic reactor ispreferably adjusted to at least 11 g/l, more in particular to between 12and 17 g/l, especially about 14 g/l. Alternatively, the electric.conductivity is preferably adjusted to between 30 and 50 mS/cm,especially about 40 mS/cm.

If the waste water to be treated with the novel bacteria containsappreciable levels of sulphide (HS⁻), the medium should contain asufficient level of methyl sources, such as methanol, methyl mercaptan,dimethyl sulphide, methylamine, or the like. If the molar concentrationof sulphide is more than twice the molar concentration of methylsources, methyl sources such as methanol are preferably added to achievesaid minimum of 1:2.

The major product of sulphide oxidation by M. sulfidovorans isthiosulphate. Usually thiosulphate is an undesirable component in wastewater. Therefore, it is preferred then to combine the use of the novelbacteria with bacteria capable of converting thiosulphate to sulphateand/or sulphur. Preferred bacteria for this purpose are those of thegenus Thiobacillus, most preferred are those of the species T.thioparus.

The novel bacteria can also be used in biological treatment of spentcaustics, more especially when it is performed with redox potentialcontrol as described above.

Redox Potential Control

The microbiological oxidation of sulphide to elemental sulphur occurseither under oxygen limited circumstances, that is at DO (DissolvedOxygen) values below, at least, 0.1 mg-L⁻¹ or under high sulphideloading rates. In the latter case, the biomass is overloaded and sulphuris formed as intermediate product. Since it is assumed that theformation of sulphur is a faster reaction than sulphate-formation, thismechanism allows the bacteria to remove the harmful sulphide at highrates.⁴ As follows from the pε-pH diagram for the SO₄ --S(s)--H₂ Ssystem, elemental sulphur is not a stable sulphur compound at pH=8.³,9At pH values below 7, elemental sulphur is formed from the oxidation ofH₂ S while in the pH range 7-11, HS⁻ would under thermodyanamicequilibrium conditions be completely oxidised to sulphate. However,since a bioreactor is a non-equilibrium system, a conceptuallymeaningful pε cannot be defined.⁹ Also the intercellular pH may bedifferent from that of the reactor suspension, resulting in differentsulphide species. As a consequence, the redox reactions which occur maydiffer from the thermodynamically predicted ones.

At loading rates below 250 mg HS⁻ L⁻¹ h⁻¹, the Thiiobacillus and similarorganisms tend to produce sulphate rather than sulphur at increasingDO-values because sulphate formation yields more energy for microbialgrowth.⁴ For reasons of environmental protection, the formation ofelemental sulphur is preferred because this compound can, in principle,be removed from the waste water and subsequently be re-used as a rawmaterial, e.g. in bioleaching processes.¹⁰ Reactors should not bedesigned to be operated under `overload conditions` for the sake ofprocess-stability. Therefore, a stoichiometrical oxygen supply isrequired to oxidise all sulphide into elemental sulphur. Since thedetection limit of currently available oxygen sensors is about 0.1mg.L⁻¹, they are not suitable as a measuring device and thereforeanother parameter should be used. A very useful alternative to controlthe oxygen supply is the application of the redox (reduction-oxidation)potential of the solution. The successful application of the redoxpotential as a control parameter for the nitrification/denitrificationprocess in biological waste-water treatment plants and its use forcontrolling the enhanced biological phosphorous removal has already beendemonstrated successfully.⁷ The redox potential is a measure of thesolution's tendency to accept or donate electrons. The thermodynamicrelation of the potential E_(H) to the composition of the solution isgenerally known as the Nernst equation:⁹ ##EQU1## for the half-reaction:n_(i) ox_(i) +n e→n_(j) red_(j). One drawback frequently mentionedconcerning the application of the redox potential is, that its value isthe result of the contribution of a mixture of dissolved components.Several redox couples may prevail and all of them contribute to themeasured, overall, redox value. However, several authors revealed theexistence of a linear relationship between the measured redox potentialand the logarithm of the hydrogen sulphide concentration in naturalenvironments.¹, 5, 6 The reason for this is that, in comparison to othersubstances, sulphide has a relatively high standard exchange currentdensity (I₀).² In a sulphide oxidising bioreactor, the measured redoxpotential therefore will predominantly be determined by the sulphideconcentration. Instead of redox potential measurements, one could alsoconsider the use of a commercially available, ion-specific, sulphideelectrode which measures the activity of the S² - ion. However, the useof such an electrode is not recommended because the S² - concentrationgreatly depends on the pH of the solution. In practice, smallpH-fluctuations will result in considerable fluctuations in the S² -concentration. The measured S² - concentration should therefore alwaysbe correlated to the actual pH-value which complicates its applicationconsiderably. The redox potential, however, is found to depend less onthe pH of the solutions. Another reason for not using ion-specificsulphide electrodes (i.s.e.) is that they are not yet available forindustrial purposes.

Materials and Methods

Reactor

A continuous-flow gaslift reactor was used with a liquid volume of 10 L.The influent consisted of tap water and a nutrient solution. The gasflow (300 L·h⁻¹) was completely recycled to prevent any release of H₂S-gas into the environment and to reach low oxygen concentrations. Pure(100% ) hydrogen sulphide gas was added to this recirculating gas streamvia Mass Flow Controllers (Brooks Thermal Mass Flowmeter, type 5850E,0-75 mL·min⁻¹). Under slightly alkaline conditions (pH=8), the H₂ S gaswas completely absorbed into the liquid phase; in the headspace no H₂ Sgas could be detected. Pure oxygen was supplied by means of twoMass-Flow Controllers (Brooks Thermal Mass Flowmeter, type 5850E, flow0-30 mL.min⁻¹ and 0-500 mL.min⁻¹). The temperature was controlled at 30°C. by a water-jacket.

Measurements and Analyses

In the reactor, the redox potential was measured with two commerciallyavailable, polished, platinum electrodes combined with an Ag/AgClelectrode as a reference (WTW, Serolyt Pt). In order to assess theeffect of the polished electrode surface, a calibration in a phosphatebuffer (KH₂ PO₄ =20 g.L⁻¹, pH=8.0) was carried out with a platinisedelectrode (platinunm black). Platinisation of a polished platinumelectrode increases the specific surface by a factor 100-1000.Consequently, reactions whereby electrons are transferred to such aplatinised electrode surface, i.e. heterogeneous reactions, may proceedfaster when the available surface area of the standard polishedelectrode is the limiting factor.² Platinisation of the electrode wasaccomplished according to the following procedure. A polishedplatinum-electrode was cleaned for half an hour in a concentrated (65%)nitric-acid solution at 70° C. After thorough rinsing with distilledwater, the electrode surface was electrochemically cleaned (10 minutes)in distilled water which was acidified with a few droplets of aconcentrated (96%) sulphuric acid solution. The direction of the current(10 mA·cm⁻²) was changed once every minute. Then, the electrode wasrinsed with distilled water and electrolysed in a 2% H₂ PtCl₆.6H₂ Osolution. Electrolysis was started for a period of 5 minutes at acurrent of respectively +10 and -10mA·cm⁻². The current was increased insteps of 10 mA·cm⁻² up to a final value of 50 mA·cm⁻². Simultaneously, ablack deposit was formed on the Pt-surface. All redox values presentedin this chapter are expressed relative to the standard hydrogenelectrode.

Sulphide was measured on-line with an ion-specific sulphide (i.s.e.)electrode which consisted of a silver-wire which was embedded insolidified resin. The silver tip was first thoroughly cleaned with adetergent solution and polished. Hereafter, the electrode was activatedby immersing it for 2 minutes in a 20% (NH₄)₂ S solution followed bythorough rinsing with tap water. In this way, an Ag₂ S-coating wascreated on the silver surface which actually served as electrodesurface. Free S²⁻ ions adsorb onto the Ag₂ S-crystal and release theirelectrons. The current was measured with a standard potentiometer. Astandard Ag/AgCl electrode was used as a reference. Thesulphide-electrode was calibrated in a double wall, air-tight vessel(V=250 mL, T=30° C.) which was filled with 100 mL of an oxygen-freephosphate buffer (KH₂ PO₄ =20 g·L⁻¹, pH=8.0). The headspace was flushedwith nitrogen before addition of a 100 mM sulphide stock solution. Thesulphide stock solution was added in steps of 0.05mL, using an automaticburette (Dosimat 665, Metrohm, Hercsau, Switzerland). The followingcalibration curve for the sulphide electrode was found:E=32.5×p(HS⁻)-446 (mV); p(HS⁻)=-log[HS⁻ ]in mg·L⁻¹, in a range of 0.5till 10 HS⁻ mg·L⁻¹. In this way 30 measurements were made and thecorrelation coefficient found was 0.99. lhe slope of the line is closeto the theoretical slope of -30 mV/p(HS⁻). The dissolved oxygenconcentration was measured using an oxygen sensor (WTW; DU 600 210). ThepH in the reactor was maintained at 8.0 (±0.1) by the addition of a 0.5M Na₂ CO₃ solution, using a custom-made pH-controller. The flow of thenutrient solution ranged from 5 mL×h⁻¹ till 25 mL×h⁻¹. The signals fromrespectively the oxygen, sulphide and redox electrodes and the mass-flowcontroller were collected via a custom made data-logger (Workshop, Dept.of Agricultural Engineering and Physics, Wageningen AgriculturalUniversity: WAU). A software PI-controller was developed using theMATLAB software-package (The Mathworks Inc., Mass.). With this programthe measuring data was also collected. The sample interval was set at 1minute for experiments with constant sulphide and oxygen loading rateswhereas the sample interval was set at 30 seconds for the experimentswith a computer-controlled oxygen dosage. Sulphate and thiosulphate weremeasured using a HPLC. The sulphide concentration was determined usingthe colorimetrical assay of Truper and Schlegel.¹¹

Results and Discussion

The relation between the redox potential and the H₂ S and O₂ loadingrates was assessed from a number of steady-state situations. Fourdifferent H₂ S-loading rates were applied, viz 50, 100, 175 and 500 mgHS⁻ ·L⁻¹ h⁻¹. At each sulphide loading rate from one up to fourdifferent oxygen loading rates were applied. The molar oxygen tosulphide consumption ratio amounted to respectively 0.38, 0.51, 0.77,1.15 or 1.54. Each steady-state was maintained for at least 24 hours. Itfollows that at a molar (O² /HS⁻)_(consumption) of respectively 0.51,0.77 and 1.15, the redox potential remained more or less constant. At anoxygen supply of less than the minimal amount required to oxidise allsulphide, i.e. when the value of the molar (O₂ /HS⁻)_(consumption) isbelow 0.5, the redox potential decreased from -142 to -161 mV. This isdue to the accumulation of sulphide (as will be discussed below). Onlyat a molar (O^(2/HS) ⁻)_(consumption) value of 1.54 a strong increase ofthe redox potential was observed. It would appear that at this ratio, ata loading rate of 50 mg HS⁻ ·L⁻¹ h⁻¹ the sulphide concentration was verylow, resulting in small absolute values of the measured redox potentialwhilst at a loading rate of 500 mg HS⁻ ·L⁻¹ h⁻¹ the redox potentialfluctuated strongly (±85 mV). This may be the result of an accumulationof oxygen ([O₂ ]=1.0 mg·L⁻¹). Since at this loading rate the biomassbecomes overloaded, sulphur is the predominant oxidation product andconsequently not all oxygen is consumed.

At a loading rate up to 175 mg HS⁻ ·L⁻¹ h⁻¹ a linear relationship isfound between the sulphate formation and the ratio oxygen/sulphideconsumption. At a loading rate of 500 mg HS⁻ ·L⁻¹ h⁻¹, however, lesssulphate is formed than the maximal possible amount due to overloadingof the biomass under these conditions, probably resulting in a reductionof the cytochrome chains of the organisms.⁸

Origin of the Redox Potential

The redox potential is an `overall` parameter which means that theoxidation and reduction of a variety of (sulphur) compounds canattribute to its value. The oxidation of sulphide to sulphur andsulphate and the reduction of oxygen in water are the major redoxchanges. The measured redox potential will thus be determined by thesereactions. The value of the measured redox potential depends inprinciple on the standard potentials (E^(H) _(O)) of the half-reactionsand the concentration of the reactants. The redox potential can only becalculated with the Nernst equation, if a thermodynamic equilibriumexists. In practice however, the measured redox potential is mainlydetermined by the compound with the highest current exchange density,i.e. the ability to exchange electrons with the platinum surface. Thismeans that the measured redox potential is kinetically determined ratherthan being dependent on the concentration of all dissolved reactants.²Sulphide (S²⁻ or HS⁻) is a compound with a relatively high currentexchange density at a platinum surface whereas oxygen has a very lowvalue.⁵ This means that in a sulphide oxidising bioreactor the value ofthe redox potential is determined by the sulphide concentration. Inthree different experiments the relationship between the redox potentialand the sulphide concentration, measured with an i.s.e. electrode, ismeasured. A linear relationship exists between the sulphideconcentration and the redox-potential. The data were collected every 30seconds from an experiment in which the sulphide and oxygen loadingrates were not in a steady-state. Because the redox electrode respondsslower to changing sulphide concentrations than the i.s.e. electrodedoes, a number of redox values were measured at each i.s.e.electrode-potential. The regression line is therefore drawn between thepoints with lowest redox values. This results in the following relationbetween the redox-potential and the sulphide concentration:E=-42*log[HS⁻ ]-158, with [HS⁻ ] in mg·L⁻¹. The values measured so far,with a normally polished redox electrode, are not in a thermodynamicequilibrium as becomes clear by comparing the results with those whichare obtained with a platinised electrode (platinum black). As followsfrom the titration curve, the calibration line obtained with aplatinised electrode has a slope of 35.0 mV/p(HS⁻) which is in closeragreement with the theoretically expected value of 30.2 mV/p(HS⁻).Laboratory research has shown that the addition of nutrient solution onthe measured redox potential has no detectable effect.

The effect of the pH on the response of both electrodes is substantial.The response of the i.s.e. electrode depends linearly on the pH, i.e. itdrops with 27 mV per pH-unit (from -445 at pH 7 to -540 at pH 10.5) dueto the increase of the concentration S²⁻ ions. Exactly the same valuehas been found by Visscher et al.¹² This value is close to thetheoretically expected value, i.e. 30 mV/(pS²⁻). For the redoxelectrode, a decrease of 14 mV/pH was found in the pH range 7-10.5, i.e.from -140 (7) to -190 (10.5).

The above results show that the measured values for the blackenedelectrode are distinctly lower than for the polished electrode, i.e.-412 mV versus -151 mV. This can be attributed to the kinetic limitationof the polished electrode; the blackened electrode accepts moreelectrons from the sulphide ions.

Real-time Control of The Oxygen Supply at Varying Sulphide Loading Rates

The ratio between the oxygen and sulphide consumption should be as lowas possible in order to minimise sulphate formation. However, the redoxpotential in steady-state should not drop below -150 mV in order toprevent the accumulation of sulphide. In a system operated under aconstant sulphide loading rate we investigated which redox levels can beattained. The redox set point for the experiment 6 amounted to -122 mV.The controller compares the measured values for the redox potential withthe desired value, i.e. the set point value. From a computer-algorithman output value for the oxygen-valve was calculated, using the so calledP and PI controllers. The redox potential reached the set point valuewithin 4 hours. This time can be reduced by a further optimisation ofthe gain factor (Kc) and time constant (τ) of the used PI-controller.

Tuning a PI-controller is a precise matter, i.e. choosing the optimalvalues for the gain factor (K_(c)) and time constant (τ). Since in oursystem the amount of biomass varies with changing ratios of oxygen tosulphide consumption and also with changing sulphide loading rates, thetime-response of the bioreactor may change as well. This means that thePI-controller also requires different sets of gain factors and timeconstants. Since the tuning of a P-controller is less troublesome than aPI-controller, some experiments with a P-controller were also performed.It was found that during the first 3.5 h of the experiment, the redoxpotential oscillated around its set point value of -122 mV whereafter itconverged to this value. However, after about 11 h of operation, thesystem apparently became unstable regarding the fact that the redoxpotential started to oscillate heavily. By repeating the same experimentat a set point value of -147 mV it was found that the measuredredox-potential converges faster to its desired value. It can beconcluded that the process can be better controlled at lower redoxvalues. Since the measured redox potential depends linearly on thelogarithm of the sulphide concentration, fluctuations around the lowerset point are therefore smaller and the process stability is thereforehigher.

It is hardly possible to maintain a redox set point of -147 mV using aP-controller although this is accompanied with large fluctuations in theoxygen flow. Apparently, the controller becomes an on/off switch. Thereason for this is, that the value for the gain factor chosen may be toohigh. Since the maximum oxygen-flow rate was truncated the system didnot become unstable. Regarding the fact that a PI-controller functionedwell in experiments with a constant sulphide loading rate, it isexpected that a combined P and PI controller is the bestcontrol-strategy for experiments with abrupt changes in the sulphideloading rate. The P-controller forces the redox potential to a valuewithin a narrow band around the set point value, e.g. -25mV<setpocontrol<+50 mV, and from then onwards a PI controller providesan almost constant oxygen flow. Such a combined controller was used forcontrolling the oxygen flow in an experiment in which the supply ofsulphide was changed stepwise.

While the experiment shows that the oxygen-flow oscillated vigorouslyfrom 0 to 17.5 mL·min⁻¹, i.e. 0-150 mg·L⁻¹.h⁻¹, a much more constantoxygen-supply rate is obtained in the experiment with the combined P/PIcontroller. It follows from the results in that the bioreactor plus P/PIcontroller responds better to an increase than to a decrease in thesulphide loading rate, i.e. the off-set from the redox set point valueat t=4 h and t=10 h is considerably smaller than at t=8 h and t=23 h.The explanation for this is that decreasing the loading rate results ina larger net-change of the redox potential than an increase. As aconsequence, the calculated deviation between measured value and the setpoint value is bigger in the former case. The results also show aperiodic change in the redox potential during the period 11-23 h, whichcannot yet be explained. In practice, a smooth fluctuation will occur,e.g. in a sinusoidal way, as shown in FIG. 1.

It is possible to control the consumption of oxygen (or another electronacceptor) over sulphide consumption under dynamic conditions, although adecrease in the loading rate results in large fluctuations of the redoxpotential. The ratio of the molar oxygen over sulphide consumption wasfound to be 0.38, which is below the stoichiometrical minimum value of0.5. The reduction of carbon dioxide to biomass is presumablyresponsible for this effect. Although a very limited amount of oxygen isconsumed, still, to some extent sulphate formation occurs (FIG. 2).Apparently, the system is incapable of completely preventing theformation of sulphate. This might be caused by small fluctuations in thedissolved oxygen concentration because at slightly higher DO-values theorganisms immediately may switch to sulphate formation.

DESCRIPTION OF THE FIGURES

FIG. 1 Effect of an imposed sinusoidal change in the H₂ S loading rateat a HRT=5 h; The redox set point was -137 mV and a combined PIPIcontroller (K_(p) =0.75, K_(c) =1.5, t_(i) =3280 s) was used.

FIG. 2 Relative production of sulphate, thiosuiphate (measured) andsulphur (calculated) during the experiment shown in FIG. 1.

NOMENCLATURE

    ______________________________________                                        τ.sub.i                                                                              integral time constant of a PI controller                          {ox}            activity of oxidisable compounds (mol · L.sup.-1)    {red}          activity of reducible compounds (mol · L.sup.-1)      DO               dissolved oxygen (mg · L.sup.-1)                    E.sub.H         half-potential (V)                                            E.sup.0.sub.H                                                                            standard half-potential (V)                                        F                 Faraday constant (9.6485 · 10.sup.5 C                         · mol.sup.-1)                                             HRT             hydraulic retention time (h)                                  i.s.e.       ion selective sulphide electrode                                 K.sub.c      proportional gain of the PI-controller                           n                 number of electrons transferred                             n.sub.i      moles of oxidisable compounds                                    n.sub.j         moles of reducible compounds                                  pε =                                                                             (E.sub.H · F)/(2.3 · R · T)             R                 gas constant (8.31 J · mol .sup.-1 K.sup.-1)       T                 Absolute temperature (K)                                    V.sub.gas        volume of the gas-phase (L)                                  ______________________________________                                    

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We claim:
 1. Process for the biological treatment of a spent causticsolution having a pH of at least 11 and containing sulphides, whereinthe solution is subjected to sulphide-oxidising bacteria in an aerobicreactor, characterised in that the sulphides are partly converted toelemental sulphur and partly to sulphate, the sulphate productionresulting in a pH lowering to between 6 and 9, by controlling the redoxpotential in the reactor at a value between -300 and -360 mV (against anAg/AgCl reference electrode).
 2. Process according to claim 1, whereinthe sulplide-oxidising bacteria comprise bacteria of the genusThiiobacillus.
 3. Process according to claim 1, wherein the redoxpotential is controlled at a value between -320 and -360 mV.
 4. Processaccording to claim 1, wherein the pH of the spent caustic solution is atleast
 12. 5. Process according to claim 1, wherein a part of theeffluent of the aerobic reactor is added to the spent caustics solutionbefore its introduction into the reactor.
 6. Process according to claim1, wherein the solution is treated in the presence of bacteria of thespecies Methylophaga sulfidovorans.
 7. Process for the biologicaltreatment of an aqueous solution containing sulphides and/or mercaptanswherein the solution is subjected to sulphide-oxidising bacteria in anaerobic reactor, characterised in that the solution is subjected tobacteria of the species Methylophaga sulfidovorans.
 8. Process accordingto claim 6, wherein the conductivity in the aerobic reactor is between30 and 50 mS/cm.
 9. Process according to claim 6, wherein the pH in theaerobic reactor is between 7 and 7.5.
 10. Process according to claim 6,wherein the solution is also subjected to bacteria of the genusThiobacillus.
 11. Process according to claim 7, wherein the conductivityin the aerobic reactor is between 30 and 50 mS/cm.
 12. Process accordingto claim 7, wherein the pH in the aerobic reactor is between 7 and 7.5.13. Process according to claim 7, wherein the solution is also subjectedto bacteria of the genus Thiobacillus.